In the related art, examples of a display element provided in a display device include an electro-optical element whose brightness is controlled with applied voltage, and an electro-optical element whose brightness is controlled with current. Typical examples of the electro-optical element whose brightness is controlled with applied voltage include a liquid crystal display element. On the other hand, typical examples of the electro-optical element whose brightness is controlled with current include an organic EL element. The organic EL element is also called an OLED (Organic Light-Emitting Diode). An organic EL display device including an organic EL element, which is a self-illumination electro-optical element, can facilitate a reduction in profile, a reduction in power consumption, an increase in brightness, and so forth compared with a liquid crystal display device that requires a backlight, a color filter, and so forth. Accordingly, the development of organic EL display devices has actively promoted in recent years.
Known drive systems for organic EL display devices include a passive matrix system (also referred to as a simple matrix system) and an active matrix system. An organic EL display device that adopts the passive matrix system has a simple structure but is difficult to increase in size and definition. In contrast, an organic EL display device that adopts the active matrix system (hereinafter referred to as an “active-matrix organic EL display device”) can easily achieve an increase in size and definition compared with an organic EL display device that adopts the passive matrix system.
An active-matrix organic EL display device has a plurality of pixel circuits formed thereon in a matrix. Each of the pixel circuits of the active-matrix organic EL display device typically includes an input transistor that selects a pixel, and a drive transistor that controls supply of current to the organic EL element. In the following, the current flowing from the drive transistor to the organic EL element may be referred to as the “drive current”.
FIG. 38 is a circuit diagram illustrating the configuration of a typical pixel circuit 91 in the related art. The pixel circuit 91 is disposed at each of the intersections of a plurality of data lines S and a plurality of scanning lines G provided in a display unit. As illustrated in FIG. 38, the pixel circuit 91 includes two transistors T1 and T2, one capacitor Cst, and one organic EL element OLED. The transistor T1 is an input transistor, and the transistor T2 is a drive transistor.
The transistor T1 is disposed between the corresponding data line S and a gate terminal of the transistor T2. The transistor T1 has a gate terminal connected to the corresponding scanning line G, and a source terminal connected to the corresponding data line S. The transistor T2 is disposed in series with the organic EL element OLED. The transistor T2 has a drain terminal connected to a power supply line for supplying a high-level power supply voltage ELVDD, and a source terminal connected to an anode terminal of the organic EL element OLED. The power supply line for supplying the high-level power supply voltage ELVDD is hereinafter referred to as the “high-level power supply line”, and the high-level power supply line is denoted by the same symbol as that of the high-level power supply voltage, namely, ELVDD. The capacitor Cst has an end connected to the gate terminal of the transistor T2, and another end connected to the source terminal of the transistor T2. A cathode terminal of the organic EL element OLED is connected to a power supply line for supplying a low-level power supply voltage ELVSS. The power supply line for supplying the low-level power supply voltage ELVSS is hereinafter referred to as the “low-level power supply line”, and the low-level power supply line is denoted by the same symbol as that of the low-level power supply voltage, namely, ELVSS. Here, the node of the gate terminal of the transistor T2, the one end of the capacitor Cst, and a drain terminal of the transistor T1 is conveniently referred to as a “gate node VG”. In general, one of the drain and the source having a higher potential is referred to as a drain. In the description of this specification, however, one of the drain and the source is defined as a drain and the other as a source. Thus, the source potential may be higher than the drain potential.
FIG. 39 is a timing chart describing the operation of the pixel circuit 91 illustrated in FIG. 38. The scanning line G is in an unselected state prior to time t1. Accordingly, prior to time t1, the transistor T1 is in the off state and the potential of the gate node VG is maintained at an initial level (for example, the level according to the writing in the immediately preceding frame). When time t1 is reached, the scanning line G is set to a selected state, and the transistor T1 is turned on. Accordingly, a data voltage Vdata corresponding to the brightness of a pixel (sub-pixel) formed by the pixel circuit 91 is supplied to the gate node VG via the data line S and the transistor T1. Thereafter, the potential of the gate node VG changes in accordance with the data voltage Vdata over a period until time t2. At this time, the capacitor Cst is charged to a gate-source voltage Vgs that is a difference between the potential of the gate node VG and the source potential of the transistor T2. When time t2 is reached, the scanning line G is set to the unselected state. Accordingly, the transistor T1 is turned off, and the gate-source voltage Vgs held on the capacitor Cst is defined. The transistor T2 supplies the drive current to the organic EL element OLED in accordance with the gate-source voltage Vgs held on the capacitor Cst. In consequence, the organic EL element OLED emits light at the brightness corresponding to the drive current.
Incidentally, an organic EL display device typically employs a thin-film transistor (TFT) as a drive transistor. In a thin-film transistor, however, variations in threshold voltage are likely to occur. Variations in threshold voltage occurring in a drive transistor provided in a display unit cause variations in brightness, resulting in a reduction in display quality. Accordingly, techniques for suppressing a reduction in the display quality of an organic EL display device have been proposed in the related art. For example, Japanese Unexamined Patent Application Publication No. 2005-31630 discloses a technique for compensating for variations in the threshold voltage of a drive transistor. Further, Japanese Unexamined Patent Application Publication No. 2003-195810 and Japanese Unexamined Patent Application Publication No. 2007-128103 disclose a technique for maintaining the current flow from a pixel circuit to an organic EL element OLED constant. In addition, Japanese Unexamined Patent Application Publication No. 2007-233326 discloses a technique for displaying an image with a uniform brightness regardless of the threshold voltage of a drive transistor or the mobility of electrons.
The techniques of the related art described above make it possible to supply a constant current to an organic EL element (light-emitting element) in accordance with the desired brightness (target brightness) even if variations in threshold voltage occur in a drive transistor provided in a display unit. However, the current efficiency of the organic EL element decreases with time. That is, even if a constant current is successfully supplied to the organic EL element, brightness gradually decreases with time. This leads to burn-in.
Therefore, if the degradation of the drive transistor and the degradation of the organic EL element are not compensated for, as illustrated in FIG. 40, a decrease in current due to the degradation of the drive transistor occurs and a decrease in brightness due to the degradation of the organic EL element occurs. Even if the degradation of the drive transistor is compensated for, as illustrated in FIG. 41, a decrease in brightness due to the degradation of the organic EL element occurs with the lapse of time. Accordingly, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-523448 discloses a technique for correcting data in accordance with the characteristics of the organic EL element OLED, in addition to a technique for correcting data in accordance with the characteristics of the drive transistor.